The disclosed invention is directed generally to display holograms, and more particularly to a display hologram structure that includes a display hologram which is optimally illuminated with a playback beam that is provided by another hologram.
Display holograms that produce pictorial images viewable by an observer have a variety of uses including vehicle instrumentation displays, pictorial displays in museums and homes, advertising displays, toys, and games, for example.
A consideration with display holograms, and holograms in general, is that they perform best when the playback illumination has the same wavefront configuration (i.e., incidence angle and angle of divergence) as the reference beam utilized for construction of the particular hologram. However, the particular application of a display hologram can make it impractical to utilize a source of playback illumination that provides the appropriate wavefront configuration. For example, an optimum pictorial hologram for a particular application might be constructed with a collimated reference beam, but the intended playback source is a point filament of a bulb. Another example is the intended playback with a converging beam of a hologram that was constructed with a diverging reference beam, for example to produce a special effect such as a pseudoscopic image. Use of a lens or mirror to produce a converging beam of a large size involves space and cost.
Pictorial holograms that are constructed with a reference beam that is near normal to the hologram recording medium (e.g., a diverging beam whose central ray is near normal), for example to reduce shadow effects or to otherwise provide a better beam for reconstruction with a given light source, provide another example the difficulty in providing a playback illumination source that produces the appropriate wavefront configuration. For such holograms, the appropriate playback wavefront would be near normal to the hologram, and the observer's head would be in the playback illumination path for a reflection hologram, and a less than optimal playback wavefront configuration would typically be utilized. For a transmission hologram configured for a near normal playback wavefront, the light source would have to be located behind the hologram, which might not be feasible in a given application, for example if the hologram is intended to be supported on a wall.